1. Field of the Invention
An aspect of this disclosure is related to a technology for delivering positional information.
2. Description of the Related Art
There exists technology for determining positions of communication terminals of users using a global positioning system (GPS). In the GPS, about 30 GPS satellites orbiting the Earth transmit radio signals indicating time. A communication terminal on the Earth including a GPS receiver receives radio signals from some of the GPS satellites and calculates the distance between the communication terminal and the GPS satellites based on differences between the time the radio signals are transmitted from the GPS satellites and the time the radio signals are received at the communication terminal. Generally, the communication terminal calculates the distances between the communication terminal and at least four GPS satellites, and determines the position of the communication terminal on the Earth based on the calculated distances.
As the size and power consumption of GPS receivers are reduced by recent technological developments, GPS receivers are installed even in small communication terminals such as cell phones that are driven by a battery.
One problem with the GPS is that GPS radio signals can hardly reach communication terminals located in an indoor space (or area). For this reason, a system different from the GPS is desired to measure positions of communication terminals located in an indoor space. As an example of such a system, an indoor messaging system (IMES) is now under the spotlight (e.g. Japanese Patent No. 4,296,302 and Quasi-Zenith Satellite System Navigation Service—Interface Specification for QZSS V1.4 provided by Japan Aerospace Exploration Agency).
A broadcasting device according to the IMES can broadcast a radio signal having the same radio wave format as the radio signal transmitted by GPS satellites. Therefore, a communication terminal can receive the radio signal from the broadcasting device using the same hardware as that used to receive the radio signal from GPS satellites with only minor changes made to reception software. The radio signal broadcast by the broadcasting device includes, instead of time information indicating time, positional information indicating the position of the broadcasting device. Accordingly, a communication terminal receiving the radio signal does not need to calculate the time differences to determine its position as in the outdoors.
In addition, there exists technology for measuring a location or a direction of a pedestrian based on a body movement of the pedestrian (e.g. Japanese Laid-Open Patent Application No. 2011-117843 and No. 2012-145457) with the communication terminal having sensors (e.g. an acceleration sensor, gyroscopic sensor, magnetic sensor, temperature sensor, and/or pressure sensor etc.). The technology is also referred to as Pedestrian Dead Reckoning (PDR).
The communication terminal with an IMES receiver uses both autonomic positioning with IMES and differential positioning with PDR, and allows positional information indicating the current position to be obtained wherever the communication terminal is. Generally, the communication terminal may calculate the positional information using the differential positioning as needed. However, the positional information is obtained by the differential positioning with frequent inaccuracies. Therefore, it is expected that the positional information is replaced with absolute positional information obtained by IMES. However, since a signal from an IMES transmitter is transmitted to a predetermined area, the absolute positional information may have a margin of error. With the related-art method, however, the communication device can not accurately determine which one of the positional information either obtained by PDR or obtained by IMES indicates a more precise current position.
An embodiment of this invention aims to allow the communication terminal to determine the positional information which precisely indicates the current position.